The present invention relates generally to a call establishment in a radio communication system comprising subscriber terminals, at least one base station, and at least one telephone exchange, whereupon a part of the subscriber connection is formed by a radio path between the terminal and the base station.
When building a telephone network, the installation of subscriber lines between the exchange and the subscriber terminals is not only expensive but also takes a considerable amount of time. One solution for saving costs is to replace the fixed subscriber loop between the exchange and a subscriber device with a radio connection. In this case the subscriber device for a fixed network is connected to a terminal which includes a radio transceiver. It converts an audio frequency signal into a radio frequency signal and sends it via a radio path to the base station, and correspondingly, converts a received radio frequency signal into an audio frequency signal.
This kind of solution can be based on a platform system which can be a cellular system as such or a modified cellular system. Together these solutions are referred to in this application as a WLL system (Wireless Local Loop System).
The principle of a WLL system based on a modified cellular system is illustrated in FIG. 1. A wireless fixed terminal T comprises a radio part equipped with an antenna and a telephone adapter 4, which connects a standard subscriber device 5 to the terminal. The user can use the subscriber device 5 just as it would be used in a normal fixed network, even though the main part of the subscriber line connection is formed by using a radio connection between the terminal device T and the base transceiver station (BTS) 2. The subscriber device can also be a normal mobile station 6 of the system to which the WLL has been adapted. Each base station can serve several subscriber devices. The, base stations are connected to a special network element 1 called an access node, which in turn is connected to a standard telephone exchange.
The access node 1 can be connected to the local exchange with an open V2 or V5.1 multiplexer connection, which uses a 2 Mb/s PCM system. If the local exchange only supports a 2-wire interface, the access node is connected to the switch with a multiplexer with the purpose of converting the V2 signaling into an analog 2-wire subscriber interface. The signaling between the access node and the base stations is the signaling of an applied mobile phone network, but it has been modified in such a manner that the functions typical of a cellular network, such as cell handover and roaming, are not possible. This means that the subscriber must stay within the assigned coverage area. The routing of incoming and outgoing calls is based on the subscriber file of the network element.
The WLL system adapted from a cellular system can be built by using the structural parts of an existing cellular system. A mobile phone system can be an analog or a digital system. In this case the signaling in the WLL system conforms; to the signaling in the cellular system, the base stations are the standard base stations of the cellular system, and the radio unit of a terminal is similar to the radio part of a mobile station, or the terminal device may be a mobile station of the mobile phone system as such. In the latter case, the terminal device and the subscriber device (phone) have been combined into one physical device.
An important network element of the WLL system is the access node. In a pure cellular system this element is a mobile services switching centre, and in an applied system it is a network element described previously in this application. The access node converts the signaling of the WLL system into signaling suitable for a fixed network and correspondingly adapts the signaling of the fixed network for the interface of the WLL network.
FIG. 2 depicts the main elements in a cellular network. It comprises a plurality of mobile services switching centres MSC. Each MSC can establish connections both to other centers and to other telecommunications networks. Several base station controllers BSC are connected to the MSC. One BSC controls a plurality of base stations BTS. A network management system collects information on the network and configures the network elements. The cellular network also includes databases. A home location register HLR stores permanent subscriber data such as subscriber service data and routing information in a visitor location register VLR. The VLR receives subscriber data from the HLR and stores it for the time when the subscriber is visiting in that VLR area.
A cellular network is well suited for a WLL network. The subscriber data in the HLR includes the information that the subscriber is a WLL subscriber. Further, information about the cell or cells where the subscriber is allowed to reside is also included in the subscriber data. These cells are called the home cell and only in these cell areas the subscribers have the right to make and receive calls. In FIG. 2, WLL terminal 21 can operate in cell A only.
It was mentioned previously in this application that signaling in the WLL system conforms to the signaling in the cellular system. Accordingly, the call establishment procedure in the WLL system follows that of the cellular system.
The line adapter in the terminal generates a dial tone when the receiver of the telephone connected to the terminal has been taken off the hook. If there is no service available, a busy tone is generated. When service is available, the dial tone is generated, and the user can start dialing. During the dialing the dialed digits, either in DTMF (Dual Tone Multifrequency) format or in pulse code format, are converted into digital format and stored in a buffer. When it is ascertained that the dialing has been completed, the contents of the buffer are sent to the transceiver. After this, a special SEND signal is given to the transceiver of the terminal to send the first message to the network. This triggers a normal call initiation procedure according to the mobile phone network used.
If the WLL system is based on the GSM system, the messages exchanged between the terminal and the network are the same as in GSM. FIG. 3 depicts messages between the terminal and the network. As the first step, the terminal requests a signaling channel by sending the message xe2x80x9cChannel Requestxe2x80x9d. Then the network searches for a free channel, and when one is located it sends the information about it to the terminal in the message xe2x80x9cImmediate Assignmentxe2x80x9d. After that the network authenticates the terminal by receiving the right response to the message xe2x80x9cAuthentication Requestxe2x80x9d. Then the network commands the terminal to cipher the signaling and the subsequent speech information, and the terminal gives notice of compliance in the message xe2x80x9cCipher Mode Completexe2x80x9d. In the ciphered mode the network sends a temporary number called TMSI (Temporary Mobile Subscriber Identity) to the terminal, which acknowledges this in the return message xe2x80x9cTMSI Reallocation Completexe2x80x9d.
It is only now that the terminal sends the dialed phone number in the message xe2x80x9cSetupxe2x80x9d. In response to that message, the network sends to the calling terminal the message xe2x80x9cCall Proceedingxe2x80x9d, which indicates that the requested access connection establishment has been initiated. After a traffic channel has been allocated, the network sends the message xe2x80x9cAssignment Commandxe2x80x9d, which informs the terminal of the traffic channel to be used. The channel assignment is acknowledged in the message xe2x80x9cAssignment Completexe2x80x9d. After these messages, the mobile switching centre starts the connection establishment to the called subscriber, and when the connection is ready, the xe2x80x9cAlertxe2x80x9d message is sent to the terminal. The user can now hear an alerting tone. If the called subscriber answers, the message xe2x80x9cConnectxe2x80x9d informs the terminal that the call has been accepted by the called subscriber. The last message xe2x80x9cConnect Acknowledgexe2x80x9d indicates that the connection between the terminal and the called number is ready to for use be used.
In WLL systems the number analysis is performed in the exchange. In the fixed line network, the dialing information is transmitted to the exchange as the dialing proceeds. The dialed numbers are analyzed and the call is established right after the exchange ascertains that the dialing has been completed. In mobile phone networks the user dials the digits of a telephone number on the keypad or uses speed dialing and then depresses a special send key on the phone in order to transmit the dialed number information to the exchange.
In WLL systems there is no special xe2x80x9csendxe2x80x9d key in the terminal. Because of the lack of a send key, one of the basic problems in the WLL networks has been how to ascertain when dialing has been completed. Several solutions are known in the art. One common solution is to use a special end character that is dialed as the last character of the string. The character e.g. # or *, which is not normally used for dialing, tells the terminal that the dialing has now been completed and the number can be transmitted. Another common solution is to use time supervision. The dialed numbers are monitored constantly, and the elapsed time starting from the last dialed digit is counted. Whenever a key is depressed, the counter is reset and time counting is restarted. Whenever the counter reaches a predetermined value, it is supposed that the dialing is finished and the number can be transmitted. A time delay limit value commonly used is six seconds.
One drawback in WLL systems of prior art is that appreciable amount of time required between the moment of dialing is started and the moment the call has been accepted by the called subscriber. The time needed for exchanging the messages of FIG. 3 could be about 5 seconds, for example. If a predetermined delay is used to detect the end of dialing, this must be around 5 seconds in order to ensure that the SEND signal is not given to the transceiver before all the digits have been dialed. Hence, the total time is could be about 10 seconds, which is considerably longer, than in a fixed network.
The objective of this invention is to shorten the call establishment time. This is achieved by an accelerated call establishment method in accordance with the independent claims.
When the user picks up the phone, i.e. lifts the receiver, the terminal detects this and generates a dial tone as normally. The user can now start dialing. But instead of waiting for the dialing to be completed, the terminal immediately sends a channel request message to the network. This triggers the exchange of normal messages with the result that a signaling channel is assigned, the authentication procedure is performed, and the ciphering mode is set. All this has been taking place during the time the user is still depressing keys to give the called subscriber""s telephone number.
The terminal is ready to send a setup message, including the called subscriber""s number to the network. Because of the dialing is still in process and the called subscriber""s number is not yet available, the terminal provides a predetermined pseudonumber for the setup message.
After the network has received the setup message, it continues the normal call establishment procedure by sending the call proceeding message and assigning a traffic channel to the terminal. But it also recognizes by the pseudonumber that a connection to the called subscriber can not be established with this number. Moreover, the pseudonumber indicates to the network that the terminal will be sending the real telephone number later. The number will arrive as DTMF digits on the assigned signaling channel.
At the same time that the network is establishing the call connection so far as possible, the terminal is receiving the dialed digits into the buffer. When the terminal recognizes that the dialing has been accomplished, it converts the digits into DTMF format and starts to send the DTMF digits one by one to the network.
The exchange in the network collects the DTMF digits and performs a number: analysis in the normal way. Based on the analysis, a connection to the called subscriber is established, and the point to point connection between the subscribers is now ready for communication.
An alternative way to send the dialed digits to the network is to send them as a string if the WLL system allows this. In the GSM system this is possible by using a data operation called Unstructured Supplementary Service Data.